Issue #107

May/Jun 2015

21st century perspective reveals that, in many ways, the advancement of recording technology has been chronologically non-linear. Rather than a tidy progression from primitive-to-sophisticated, we could easily characterize the developments as an evolution from one set of priorities to another. The best tool for one job might be the result of cutting-edge research and development while the best tool for another might incorporate half-century-old technologies. Depending on the individual demands of a project, you will likely find at least an occasional need for equipment designed around certain classically enduring ideals.

The Coil Audio CA-286 PS6 six-channel modular preamp and line-amp setup takes clear styling and technological cues from tube consoles of the first golden era of high fidelity recording. With this aesthetic and functional link established, perhaps the most effective way to assess its position in a modern studio workflow will be to examine where it aligns with (and where it departs from) the more sought-after gear from this era.

Considering the better legacy gear as a benchmark, the first point of comparison should concern reliability and serviceability. Classic tube equipment typically featured construction that was not only bulletproof but eminently field-serviceable; in a bygone era of large union-scale orchestras, any time-consuming failure or service could spell expensive disaster. With a handsome unit that's not only solidly built but also quite modular in nature, Coil Audio tips its hat to this mid-century, quasi-military design ethos. Individual channels are secured with two thumbscrews and slide out easily for service, terminating in military-grade Cinch connectors. Input and output transformers are pluggable via standard octal tube sockets. At the component level, the highest quality is in evidence throughout, from military-spec passive components to rugged NOS Valvo EF86 tubes. The separate 2RU-height rackmount power supply has individual switches for filament and B+ voltages, allowing gentle power-on procedures to maximize tube life. Dual fuses feature caps that illuminate in the event of a fault — a nice touch, should troubleshooting become necessary. From a durability and serviceability standpoint, Coil Audio seems to have thought of everything — and to have gotten it all right. The uncompromising level of quality here is very impressive.

In addition to extreme reliability, equipment aspiring to mid-century utilitarianism should have good sound as the other primary concern — far above cost, size, or convenience. Coil Audio seems to have likewise understood this with their development of the CA-286, as each component seems to have been carefully selected with sonic intent. At the heart of any piece of tube equipment lie the tubes themselves, so inclusion of the aforementioned NOS Valvo EF86s (which, subjectively, are in the top echelon of EF86 types) is notable. Also sonically important are Coil Audio's own transformers, which come in a few flavors, and are wound in the USA to exacting standards. With the help of an experienced manufacturing partner, Coil Audio reverse-engineered some of its favorite vintage transformers by Foster and Peerless in order to duplicate as closely as possible their metallurgy, winding geometry, and consequent sonic signatures. The attention to detail extends to the passive components, for which classic and sonically- reputable NOS paper-in-oil capacitors, Allen-Bradley carbon- composition resistors, and 2 watt PEC vintage-style hot- molded carbon potentiometers (made on old Allen-Bradley tooling) are employed. In a final nod to an apparent "there are no unimportant parts" design philosophy, internal connections are made with name-brand wire.

Where the CA-286 departs somewhat from the postwar-era zeitgeist is the feature set. Basic functionality includes everything you'd expect from a tube preamp: an output fader, a polarity switch, and a five-position "input" switch (four levels of attenuation plus line-level). Balanced H-pad attenuators are used here to keep impedances constant regardless of how much attenuation is employed — an elegant touch that attests to a sonically uncompromising design philosophy. However, in contrast to the rigid-yet- elegant simplicity of classic mic and line amps, the CA-286 allows the end-user to get under the hood and tweak several seemingly-fundamental aspects of the basic design, including transformer specification and the particulars of the amp's negative feedback circuit.

Negative feedback is a very common design element in which a portion of an amplifier's signal is polarity-inverted and "fed back" into an earlier stage. Essentially an error-correction mechanism, the objective is to counteract any irregularities and to encourage flatter response, lower distortion, and greater stability. Subjectively (and at the risk of oversimplification), tube amplifiers employing a large amount of negative feedback tend to sound more damped and controlled, with measurably lower distortion and flatter frequency response. Mitigating or dispensing with the feedback, on the other hand, allows a bit more harmonic distortion and lets the naked sound of the tube shine through. Every design is unique, but often a tube amp with little-to-no negative feedback can feel lively and exciting.

Coil Audio gives users a great degree of control over this parameter with a set of highly-interactive controls. A front-panel "NF" knob allows the user to vary the amount of negative feedback employed. Stepped "Hi" and continuously-variable "Low" tilt controls afford the ability to shape the frequency response of the negative feedback circuit, impacting how and where the feedback interacts with the tube's raw output. Important to note: Hi and Low controls are not a conventional equalizer. They influence the timbre within the amplifier itself by acting on the negative feedback loop. With NF past noon or so, turning Low clockwise pushes bottom end forward in a way that, while subtle, feels different from boosting bass with a conventional EQ. Although novel in a mic preamp, this technology will feel familiar to some; the "Presence" control on old Fender and Marshall guitar amps boosts top end using a very similar principle.

The interactivity of these controls makes single-variable tuning unfortunately impossible, which lengthens the learning curve. In addition to changing sonic character, a clockwise twist of NF reduces the overall gain and heightens the impact of the Hi and Low controls. Conversely, with NF fully counterclockwise, gain is increased and Hi and Low are rendered ineffective. Assessing the impact of small adjustments can be a familiar mind game — does it sound better when I turn this knob, or just louder? Practical realities of the circuit design indicate that it had to be this way, but I did find myself wishing that the controls could've been made more intuitive as individual elements.

In this quibble lies the crux of what will be for some a relatable bias. While more adjustable parameters theoretically affords greater flexibility, in tracking-session practice, I tend to lean toward simpler things that just work, allowing me to focus on keeping creative momentum and capturing stellar performances with a minimum of comparison-minded fine tweaking. In this way, the CA-286 departs a bit from the tube-era simplicity that is my personal preference, though others may appreciate having finer control over the sonic character. For those who work the way I do, though, the timbre- shaping capabilities may prove more useful if using the CA-286 as line amps or inserts in a mix situation, where logistical realities afford greater opportunity to tweak and carefully consider minute changes in tone. Regardless, this is a product whose effectiveness will be maximized when the user has gained experience with it — not exactly a foolproof or "visitor-friendly" piece of kit, but certainly something that a slow-burning love affair could be kindled with. There is creative sound-shaping potential present in the NF circuit if users are willing and inclined to roll up their sleeves and dig in.

Upon first unpacking the units at The Bunker Studio in Brooklyn, a couple of our colleagues took the CA-286 PS6 for its first spin. Primarily due to being on the beginning of that learning curve, it took a while to realize that NF at minimum is a good starting point for drums or other sources calling for a present, in-your-face character. Pushing NF clockwise seems to relax things, darkening the tone a touch and creating some illusion of distance. On male vocal using a Neumann U 47, I again preferred NF mostly-to-fully counterclockwise; set up thusly, the CA-286 afforded a silvery, large, forward sound on the voice that was quite impressive. An overdub of a clean electric archtop guitar part fleshing out an understated arrangement worked well with NF pushed farther to the right, and the Hi tilted down a notch. This allowed the guitar to very naturally sit further back in the soundstage without having to make it overtly reverberant. On a DI'ed electric bass, I likewise preferred NF clockwise a bit, taking advantage of the inherently more muted texture and allowing use of the Low tilt. When the NF setting permits its use, this control is subtle but natural. The slope feels gentle, and even with the control at maximum boost or cut, there is a refreshing absence of what one might call "EQ sound." The same holds true for the Hi tilt, which really felt nice pushing the tops on a pair of darker vintage K Zildjian Istanbul hi-hats.

Personal workflow quibbles aside, it's refreshing to see a new company enter the field with this level of attention to detail, an apparent dedication to durability and sonic excellence, and a willingness to step into some unconventional design territory. Coil Audio's variable and tilt-tweakable negative feedback design is novel and will appeal to those who love to dig for finer control over aspects of the fundamental design. Likewise, the plug-in transformer options will be hours of fun for those who like to experiment with nuance down to the component level. In an era that finds many treating "tube rolling" like a hobby, there will most certainly be an audience for these features.

The other edge of that sword is that fundamental tweakability is not for everyone — visiting freelance engineers might be confused by the interactivity of the feedback-related controls, may expect something different from controls labeled Hi and Low, or simply might not luck into the best sound for a given source the first time. "Option overload" could easily take hold if you're the type who would rather a signal chain just sound great when you twist one knob to make the mic louder, leaving things like negative feedback to the designers.

Still, even if that describes you, the Coil Audio products might be worth your time to investigate. Sound and feature preferences are ultimately subjective in every case, but reliability and serviceability are much less so — and Coil Audio knocks it out of the park in these respects. I solemnly swear that this is not your typical-review "feels solid" or "seems to be built like a tank" trope. Even the most informed observer would have to acknowledge that these are built 100% right down to the component level. This is a company that cares about quality, and even if you're notoriously picky about minutiae, you will find their parts selection and construction techniques to be unimpeachable.

As always, quality costs, but if the full six-channel rack is a bit rich to start, Coil also offers a two-channel unit as well as the ability to buy the six-channel chassis and load it one module at a time. And finally, though it may seem like a superficial concern, I'd be remiss if I failed to point out that the design of this unit is gorgeous. More importantly, the CA-286 is a well-built device that, while it has a bit of a learning curve, will reward you with stellar sound provided you take the time to set it up optimally for a given source.

Tape Op is a bi-monthly magazine devoted to the art of record making.

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